1. Field of the Invention
The present invention relates to an asynchronous machine having a rated power of at least 1 MVA, comprising a laminated stator core which supports a multiphase AC winding and a rotor with a rotor core, surrounded by the stator core, and winding slots running axially along the entire rotor core, each of said winding slots being provided with wall portions facing each other and converging towards an axial central plane and containing a number of winding rods included in a squirrel-cage winding and adapted in shape to the slot walls, the winding rods being sufficiently short-circuited at each rotor end by means of an annular winding portion.
2. The Prior Art
More particularly, the invention relates to a machine construction which is particularly suitable when the current density of the winding and the product of the rotor diameter and the rotor speed is especially great. Based on calculations it can be proved that a ring included in a rotor, for example, a short-circuiting ring in a rotor with a squirrel-cage winding or a retaining ring in a turbo-generator, may be the element which, at high speed, sets an upper limit to the permissible rotor diameter. In a squirrel-cage motor, the starting current, for example, may produce such a great generation of heat in the short-circuiting rings that their strength is reduced, which, in turn, results in deformation and unbalance when the motor reaches full speed if the centrifugal forces are too great.
In two-pole turbo-generators with a speed of about 3000 r.p.m. and a power of more than 1000 MVA, the rotor diameter, because of problems as regards the tensile properties of the retaining rings, must often be chosen smaller than is desirable from other points of view. Calculations concerning a retaining ring show, for example, that its own mass only, with an increasing diameter, causes such a high tangential stress therein that the ability of the ring to take up also centrifugal forces from the coil ends of the rotor becomes zero if the diameter of the rotor exceeds 1500 mm.